This invention relates to an eccentric gear for transferring a torque or a force between rotatable elements, one of which is driving and another is driven, comprising two co-operating gear rings with differently large pitch diameters, of which a first, outer ring of a largest diameter has a certain number of inwardly directed teeth, and a second, inner ring has a smaller number of outwardly directed teeth, of which a certain number engage with the outer gear ring by the fact that the inner gear ring is eccentrically movable and rollable relative to the outer, more specifically by the fact that the inner gear ring is connected to an elongated body, whose geometrical axis of rotation is oblique relative to a geometrical axis of symmetry through the center of the outer gear ring and which during the generation of the inner gear ring against the outer gear ring is submitted to a nutating motion, during which the axis of rotation of the body moves as a generatrice along the envelope surface of an imaginary cone, viz. around a nutation point that is axially distanced from the gear rings, said body being, in a region that is substantially equally distanced from the gear rings as the nutation point, co-rotatively connected to the driven element, and the gear change between said elements being dependent on the total number of teeth in the respective gear rings.
An eccentric gear of the type described above has been previously disclosed in WO 96/05451, which claims priority from SE 9402701-8. A general advantage with eccentric gears of this type is that they in one single step make possible large gear changes. A special advantage with the specific eccentric gear disclosed in WO 96/05451, is that it does not need any own output shaft in bearings, but may utilize the own bearing of the driving element. Then, the driven element is provided with the one half of an arch-tooth coupling, whose other half is included in an elongated body that carries the inner gear ring. This arch-tooth coupling eliminates the risk of either the gear or the driven element being loaded with dangerous breaking loads in bearings; something that guarantees that the adjustment of the gear relative to the driven element does not become critical.
However, the embodiments exemplified in said document are of such a nature that they shall be capable of transferring comparatively large torques to the driven element. Due to this reason, the elongated body that carries the inner gear ring has a comparatively large moment of inertia by being strongly constructed. Thus, according to a major number of embodiments, the body is tubular and made of massive metal. Therefore, the known embodiments have turned out to be less appropriate for such practical applications, where the requirements of high torques are moderate, but the requirements of a short length of construction, constructive simplicity and a silent operation are the larger.
The present invention aims at removing the above-mentioned shortcomings of previously known eccentric gears and creating an improved eccentric gear. Thus, a primary object is to create an eccentric gear with a minimal length of construction. Another object is to create an eccentric gear of a low sound level. Still another object is to provide an eccentric gear of a simple mechanical construction, which may be manufactured in a rational and cost-effective way.
According to the invention, at least the primary object is achieved by the characteristics that are defined in the characterizing clause of claim 1. Preferred embodiments of the invention are further defined in the dependent claims.
In FIG. 8 of the above-mentioned WO 96/05451, an embodiment is exemplified that comprises a central axle journal which at one end is co-rotatively connected to a sleeve-shaped part of a gear-ring-carrying body, and at the other end is connected to a wheel with a toothing for engagement with a cable or chain. However, in this case the journal is not introduced into any elongated hole in the driven element, nor is it co-rotatively connected to it at a point situated deeply within the driven element.